Meanwhile, the Syrian Observatory for Human rights, which tracks developments in the war and is opposed to the Assad regime, reported that IS had pushed Kurdish fighters out of 60 villages on the Turkish border.

Esmat al-Sheikh, head of the Kurdish forces defending Kobani, said fierce clashes continued to the east, west and south of the town, which Turkey adjoins to the north.

He said IS fighters armed with rockets, artillery, tanks and armoured vehicles seized from the Iraqi army in Mosul had advanced to within 20 kilometres of Kobani.

Mr Sheikh said women, children and the elderly had been killed on roads in the area.

"They took women, girls, small children and youths as hostages," he said.

Ocalan Iso, deputy head of the Kurdish forces in Kobani, said the Kurds had evacuated 100 nearby villages because of IS shelling.

"These villages have become battlefields. They do not control them," he said.

Turkey has re-opened its border with Syria to allow in families who are escaping the fighting.

Women, children fleeing 'to escape death'

Machine gun fire and shelling could be heard from miles away as Turkish soldiers escorted about 4,000 Syrian Kurds, mostly women and children, over the border at Dikmetas, a Turkish village 20 kilometres along the frontier from Kobani.

One woman was hospitalised after stepping on a landmine.

When our brothers from Syria and elsewhere arrive at our borders to escape death, without discrimination over religion or sect, we take them in and we will continue to take them in.

Turkish prime minister Ahmet Davutoglu

Turkish security forces at one point fired tear gas and water cannons to disperse a group of about 100 people as they sought to cross into Syria in an apparent protest against the tight border security.

Turkish prime minister Ahmet Davutoglu, who has said the priority is to give aid on the Syrian side of the border, said he had given the order for the Syrians to be let in after receiving information that 4,000 had arrived seeking shelter.

"When our brothers from Syria and elsewhere arrive at our borders to escape death, without discrimination over religion or sect, we take them in and we will continue to take them in," he told reporters during a trip to Azerbaijan.

An official from the Turkish relief agency AFAD said they had been preparing for an influx for days and tents had already been sent.

Turkey is trying to persuade the United States of the need to create a "buffer zone" inside Syria, Turkish officials said, a safe haven on the border likely to require a foreign-patrolled no-fly zone where displaced civilians could be given aid.

Talks with US officials were continuing on the idea, a senior Turkish official said.

Bringing terrorists to justice

Meanwhile, the US Senate has unanimously approved a bill providing up to $US10 million for information leading to the arrest or conviction of people involved in executing two American journalists murdered by jihadists.

Slain journalists remembered

"One way we can honour the memories of James Foley and Steven Sotloff is to bring their evil murderers to justice, which this measure will help do," senator Marco Rubio said.

Mr Rubio, from Sotloff's home state of Florida, said it was important to send the message "that the United States will work tirelessly to ensure that the deaths of these beloved journalists do not go unpunished".

Extremists from IS, which has taken over swathes of territory in Iraq and Syria, published graphic videos showing the beheadings of Foley and Sotloff.

"Our country lost two of its own, and we must do everything possible to bring the terrorists who committed these atrocities to justice," senator Jeanne Shaheen said.

The measure would authorise the secretary of state to provide the money under the State Department's Rewards for Justice program, which was established in 1984.

The bill would need to pass the House of Representatives before it goes to president Barack Obama for his signature.